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Abstract We present the first multiepoch broadband radio and millimeter monitoring of an off-nuclear tidal disruption event (TDE) using the Very Large Array, the Atacama Large Millimeter/submillimeter Array, the Allen Telescope Array, the Arcminute Microkelvin Imager Large Array, and the Submillimeter Array. The off-nuclear TDE AT 2024tvd exhibits double-peaked radio light curves and the fastest-evolving radio emission observed from a TDE to date. With respect to the optical discovery date, the first radio flare rises faster thanF_ν ∼ t⁹at Δt = 88–131 days and then decays as fast asF_ν ∼ t⁻⁶. The emergence of a second radio flare is observed at Δt ≈ 194 days with an initial fast rise ofF_ν ∼ t¹⁸and an optically thin decline ofF_ν ∼ t⁻¹². We interpret these observations in the context of a self-absorbed and free–free absorbed synchrotron spectrum, while accounting for both synchrotron and inverse Compton cooling. We find that a single prompt outflow cannot easily explain these observations and that it is likely that either there is only one outflow that was launched at Δt ∼ 80 days or there are two distinct outflows, with the second launched at Δt ∼ 170–190 days. The nature of these outflows, whether sub-, mildly, or ultrarelativistic, is still unclear, and we explore these different scenarios. Finally, we find a temporal coincidence between the launch time of the first radio-emitting outflow and the onset of a power-law component in the X-ray spectrum, attributed to inverse Compton scattering of thermal photons. 

Abstract Several technosignature techniques focus on historic events such as SN 1987A as the basis to search for coordinated signal broadcasts from extraterrestrial agents. The recently discovered SN 2023ixf in the spiral galaxy M101 is the nearest Type II supernova in over a decade, and will serve as an important benchmark event. Here we review the potential for SN 2023ixf to advance ongoing techonsignature searches, particularly signal-synchronization techniques such as the “SETI Ellipsoid” that identifies over time stars that could transmit signals after observing a supernovae event. We find that more than 100 stars within 100 pc are already close to intersecting this SETI Ellipsoid, providing numerous targets for real-time monitoring within ∼3° of SN 2023ixf. We are commencing a radio technosignature monitoring campaign of these targets with the Allen Telescope Array and the Green Bank Telescope. 

As radio spectrum becomes increasingly scarce, coexistence and bidirectional sharing between active and passive systems becomes a crucial target. In the past, spectrum regulations conferred radio astronomy a status on par with active services, thereby protecting their extreme sensitivity against any harmful interference. However, passive systems are likely to lose exclusive allocations as capacity constraints for active systems increase. The resulting increase in ambient radio frequency noise from various terrestrial and non-terrestrial emitters can only be mitigated with informed collaboration between active and passive users. While coexistence using time-division spectrum access has been proposed in the past, a more dynamic approach following the CBRS sharing principle promises greater spectral occupancy and efficiency, enabled by a spectrum access system capable of constantly monitoring the ambient RF environment. Instead of simply minimizing the potential for any ”harmful” interference to passive users, the goal is to use good engineering to enable sharing between active and passive users. To this end, this research created a Software Defined Radio (SDR)-based testbed at the the Hat Creek Radio Observatory to quantitatively characterize the radio-frequency environment, and flag potential sources of radio frequency interference in the vicinity of the Allen Telescope Array. Sensor validation was carried out via data analysis of I/Q data collected in well-characterized RF bands. Results so far from ground and drone-based surveys are consistent with the expected sources of interference, based on both the deployment of static RF transmitters in the Hat Creek/Redding area as well as the interference detected in telescope observations themselves. 

Abstract Recently the James Webb Space Telescope performed near-infrared spectroscopic observations of the atmosphere of a potential Hycean exoplanet, K2-18 b. These spectra provided evidence of methane and carbon dioxide in its atmosphere, along with a possible line attributed to biomarker dimethyl sulfide. In this work, we present triggered narrow-band radio observations of K2-18 b conducted using the Allen Telescope Array over 3–10 GHz, in search of signs of artificially produced radio emissions (technosignatures). We do not find any spatially isolated signals in the direction of K2-18 b, establishing lower and upper limits on the equivalent isotropic radiated power (∼10¹³–10¹⁶ W) of potential extraterrestrial transmitters between 3 and 10 GHz. This study emphasizes the importance of ongoing observations to further explore K2-18 b’s potential as a candidate for the detection of technosignatures. 

ABSTRACT FRB 20220912A is a repeating Fast Radio Burst (FRB) that was discovered in Fall 2022 and remained highly active for several months. We report the detection of 35 FRBs from 541 h of follow-up observations of this source using the recently refurbished Allen Telescope Array, covering 1344 MHz of bandwidth primarily centred at 1572 MHz. All 35 FRBs were detected in the lower half of the band with non-detections in the upper half and covered fluences from 4–431 Jy-ms (median = 48.27 Jy-ms). We find consistency with previous repeater studies for a range of spectrotemporal features including: bursts with downward frequency drifting over time; a positive correlation between bandwidth and centre frequency; and a decrease in sub-burst duration over time. We report an apparent decrease in the centre frequency of observed bursts over the two months of the observing campaign (corresponding to a drop of 6.21 ± 0.76 MHz per d). We predict a cut-off fluence for FRB 20220912A of Fmax ≲ 104 Jy-ms, for this source to be consistent with the all-sky rate, and find that FRB 20220912A significantly contributed to the all-sky FRB rate at a level of a few per cent for fluences of ∼100 Jy-ms. Finally, we investigate characteristic time-scales and sub-burst periodicities and find (a) a median inter-subburst time-scale of 5.82 ± 1.16 ms in the multi-component bursts and (b) no evidence of strict periodicity even in the most evenly spaced multi-component burst in the sample. Our results demonstrate the importance of wideband observations of FRBs, and provide an important set of observational parameters against which to compare FRB progenitor and emission mechanism models.